Self-assembly of polymers promises to vastly improve the properties and manufacturing processes of nanostructured materials, since self-assembly is highly parallel, quite versatile, and easy to implement. Especially promising are novel compounds known as block copolymers, formed by two chemically different polymers that are linked together. Guided patterned arrays have been produced using electron-beam lithographic techniques or nano-imprint lithography, but these methods are painstaking, and they have not yet been able to produce perfect surfaces over large areas. Recently, a group of researchers used faceted surfaces of commercially available sapphire wafers to guide the self-assembly of block copolymer microdomains. Grazing-incidence small-angle x-ray scattering (GISAXS) at ALS Beamline 7.3.3 verified the arrays' quasi long-range crystalline order over arbitrarily large wafer surfaces. It's expected that this new method of producing highly ordered macroscopic arrays of nanoscopic elements will revolutionize the microelectronic and storage industries and perhaps others, such as photovoltaics.

Self-Assembling Molecular Legos

Tom Russell (UMass) and Ting Xu (UCBerkeley) had separately been working on understanding, and ideally controlling, the hierarchical self-assembly of complex systems. In their ideal scenario, one would be able to throw a bunch of nanometer-scale Legos with all the desired properties into a box, shake it up, and have the molecular Legos assemble themselves into functional units without any complicated chemistry or processing. During the course of a long-distance phone call, Russell and Xu saw in a flash of insight that atomic order could be translated to larger scales by using surface ridges of a base crystal to guide the assembly, like using the corrugations in cardboard to direct how closely-packed marbles will order themselves. The density achievable with the technology they subsequently developed could be high enough to allow the contents of 250 DVDs to fit on a surface the size of a quarter. The new technique for guiding self-assembly should not only increase data storage volume, but will save months in manufacturing and open up vistas for entirely new applications.

An overhead view (top) of cylindrical block-coploymer structures, consisting of a central polymer (blue) linked to a surrounding polymer (red). An atomic-force microscope image (center) shows the densely packed cylinders, dark in the center. The varying distance from the crystal surface is indicated by superimposed dark and light stripes. The side view diagram (bottom) shows how the cylinders arrange themselves along the ridges of the crystalline facets.

Crystals are naturally atomically ordered, and large single crystals are commercially available for many materials, including silicon and sapphire. The researchers knew that reconstructed surfaces of these single crystals produce facets—saw-toothed topographic patterns that are in registry with the underlying crystal over large distances. They hypothesized that they could use this phenomenon to transfer the ordering of a single crystal at the atomic level to a block copolymer assembly on the scale of tens of nanometers.

The initial steps were simple. First they chose large single crystals of sapphire cut along specific crystallographic planes. The featureless cut crystal was then heated to very high temperatures, 1300 to 1500 °C and annealed for 24 hours. During heating and annealing, atoms exposed between the edges of the cross-cut planes rearranged themselves in the lattice, with the result that the surface of the crystal reconstructed itself as a series of parallel ridges. On this serrated surface, block copolymer thin films were allowed to self-assemble into nanoscopic cylinders standing upright from the surface of the sapphire. The structures were analyzed by atomic force microscopy and by grazing-incidence small-angle x-ray scattering (GISAXS) performed at Beamline 7.3.3—the first demonstration that GISAXS can provide a metric to characterize ordering on the nanoscopic scale over macroscopic distances.

To generate long-range arrays of densely packed cyclindrical domains, the researchers began with single crystals of sapphire cut at an angle to the crystalline planes. The cut crystal was heated to over 1300 °C and annealed in air for 24 hours to form saw-tooth patterns of parallel facets. A thin film of block copolymers was applied to the surface; chemical annealing produced an array of highly ordered, densely packed cylindrical domains extending across several square centimeters of the crystal. At bottom, atomic-force microscope images of the surface and copolymer array show the different stages.

In microscope images, the cylinders appear as hexagonally packed dots with arrangements readily directed by the parallel ridges of sapphire. Each cylinder in the array is a mere three nanometers in diameter, but the array extends over several square centimeters without a flaw, unguided by any pre-existing lithographic pattern. At first the researchers were concerned that defects in the sapphire substrate could destroy the order of the array. What they found was just the opposite. Although there are indeed many dislocations in the surface of the annealed crystal, the self-assembling film of copolymers maintained its perfectly hexagonal organization right over them, covering an area of a few square centimeters. From the atomic structure of the crystal lattice to nanometer-scale copolymer structures to centimeter-scale arrays is a wide span, amounting to perfection maintained over many orders of magnitude.

The achievement of a 10-terabit array of block copolymers formed in a single step on oriented crystal facets offers immediate practical promise. By treating the film of polymer structures with a solvent, the core polymer at the center of each cylinder is easily removed. The resulting thin film is a nanometer-sized sieve of a kind that could be used as a template for data storage or nanowires or other ordered nanoscopic structures for use in electronics or other devices. Future possibilities also include working with synthetic peptides and artificial proteins, as well as with block copolymers and nanoparticles, to build new functional materials based on molecules designed with novel electronic, photonic, and biological properties.